Atoms, Molecules and Stoichiometry

Question 1

What is the unified atomic mass?

Answer 1

One-twelfth of the mass of a carbon-12 isotope

The unit for this value is u

Question 2

What is relative atomic mass?

Answer 2

The ratio of the average mass of the atoms of an element to one unified atomic mass unit

• As it is a ratio, it has no unit
• The average mass of the isotopes relative to their abundances must be calculated to find the average mass

Question 3

What is relative isotopic mass?

Answer 3

The mass of an isotope relative to one unified atomic mass unit

Question 4

What is relative molecular mass?

Answer 4

The ratio of the average mass of a molecule to one unified atomic mass unit

Question 5

What is relative formula mass?

Answer 5

The ratio of the average mass of a single unit of a compound (usually one containing ions that does not form discrete molecules) to one unified atomic mass unit

Question 6

What is the Avagadro constant?

Answer 6

• 6.022×10²³
• It is the number of atoms/molecules that a single mole of an atom/molecule consists of
• One mole of a substance will be the relative atomic/molecular mass of that substance in grams

Question 7

What are the formulae for these ions?

Silver(I)
Ammonium
Zinc(II)
Hydroxide
Nitrate
Sulfate
Carbonate
Hydrogen carbonate
Phosphate

Answer 7

Ag⁺
NH₄⁺
Zn²⁺
OH⁻
NO₃⁻
SO₄²⁻
CO₃²⁻
HCO₃⁻
PO₄³⁻

Question 8

What is water of crystallisation?

Answer 8

• Water that is part of the structure of a hydrated crystal
• This water is represented by a dot, e.g. CuSO₄.5H₂O. The number before the water is the number of moles of water relative to the number of moles of the crystalline compound
• Compounds in crystals without water are called andhydrous compounds
• Water can be removed and added to a crystal via a reversible reaction